Peanut-Like Hematite Prepared by a New Facile Hydrothermal Process for Removal of As(V)
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Peanut-like hematite has been prepared by a new facile hydrothermal method and applied in the adsorption removal of As(V). The structural features of the as-prepared hematite were characterized systematically by X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller, scanning electron microscopy, energy-dispersive X-ray spectroscopy mapping, Fourier transform infrared spectroscopy, and transmission electron microscopy. Results showed that the morphologies of hematite could be tuned to spindle-like, oval-like, and cantaloupe-like shapes by adjusting the hydrothermal conditions. The peanut-like hematite formation followed a five-step route. At pH = 3, the adsorption amount of As(V) over peanut-like hematite reached 13.84 mg/g, and the adsorption kinetic process corresponded to the pseudo-second-order kinetic model. The peanut-like hematite also showed partial selectivity over As(V) in the hydrosphere. This method can be a reference for the preparation of other architectural metal oxide materials.
KeywordsPeanut-like hematite Arsenic adsorbent Nanoparticle
Arsenic contamination has led to serious environmental problems because of its severe toxicity and risk to human health . Thus, removing arsenic from aqueous environment is of critical importance. Recently, significant efforts have been devoted to removing arsenic pollutants from water systems by various strategies, including coagulation, adsorption, and reverse osmosis . Among these strategies, adsorption is the simplest and most effective method for arsenic removal. Adsorption efficiency is highly dependent on adsorbent characteristics. Hence, high-efficient adsorbents must be urgently developed.
Hematite is an attractive and environment friendly adsorbent owing to its chemical stability, easy fabrication, and unique optical and electric properties; it is widely applied in adsorption, chemical catalysts, lithium-ion batteries, gas sensors, and electrode materials . Given the specific interactions between hematite and oxyanions of arsenic species, hematite can be used as an adsorbent to remove arsenic . Adsorption efficiency is related to the specific surface area, morphology, and surface groups of adsorbents . Different hematite morphologies, such as dots , rods , wires , arrays , tubes , belts , disks , rings , and flower-like shapes , have been obtained. Sugimoto et al.  have synthesized hematite particles by the sol–gel method. Jia et al.  have synthesized hematite with a controllable size. However, the synthesis processes are tedious, and the adsorptive efficiency of arsenic remains unsatisfactory [17, 18]. Thus, a facile method must be developed to synthesize diverse hematite nanostructures.
In this study, we report for the first time a new facile hydrothermal process using 5-sulfoisophthalate acid sodium salt (5-SSIPA) to synthesize peanut-like hematite and remove As(V). The possible formation process of such peanut-like hematite was put forward, and the efficiency and selectivity for the As(V) removal of the peanut-like hematite were also studied.
All procured chemicals were of analytical grade and used without further purification. Typically, 0.02 mol FeCl3·6H2O and 0.02 mol 5-SSIPA were dissolved in 60 mL deionized water in a 100 mL Teflon-lined stainless steel autoclave and heated at 190 °C for 6 h. Afterward, the autoclave was cooled to room temperature naturally, and a reddish brown powder was obtained by centrifugation and washed at least thrice with deionized water. Then, the reddish brown powder was dried by a freeze dryer and stored in a glass vial.
The morphology and distribution of oxygen and iron elements were demonstrated by scanning electron microscopy (SEM). The valence states of Fe were analyzed by X-ray photoelectron spectroscopy (XPS). The crystal morphology was demonstrated by transmission electron microscopy. The phase reflection was analyzed by X-ray diffraction (XRD) with CuKα (λ = 0.15406 nm) radiation. The infrared optical properties were demonstrated by Fourier transform infrared spectroscopy (FT-IR). The Brunauer–Emmett–Teller (BET) surface area and pore size distribution were analyzed by N2 adsorption–desorption isotherm.
The peanut-like hematite with a concentration of 1 g/L was equilibrated in 30 mL NaCl solution with concentration of 0.1 mol/L for 2 h. Then, As(V) and HCl (0.1 mol/L) were added into the solution with an initial concentration and pH of 5100 mg/L and 3.0, respectively. The solutions were placed in a water bath shaker at 160 r/min and 25 °C. A total of 1 mL filtrate was obtained from the initial solutions at concentrations of 5 and 100 mg/L after a regular time (5, 10, 15, 25, and 35 min) and 12 h, respectively. As(V) concentrations were tested by atomic fluorescence spectrometry (Rayleigh, China) as referred to the work of Liu et al. .
Results and Discussion
Combining the XRD patterns in Fig. 5a and SEM images in Fig. 5b–f, we proposed and simulated the process of a five-step route consisting of nucleation, aggregation, phase transition, anisotropic growth, and ripening. Small green rust seeds (Fe(OHCl)2.55) were generated by nucleation and crystal growth (step 1, Fig. 5b, 1 h). These seeds aggregated to form larger rod-like structures and transformed into iron hydroxide, goethite, and iron oxide hydroxide (step 2, Fig. 5c, 1.5 h) and then continuously aggregated and partly transformed into oval-like hematite (step 3, Fig. 5d, 2 h). At the same time, the proceeding phase transition led to the generation and deposition of hematite. As the interfacial energy between the nanoparticles and solution was higher than that between the nanoparticles and oval-like structure, the oval-like product would continuously grow anisotropically, and cantaloupe-like hematite was formed (step 4, Fig. 5e, 3 h) . With prolonged time, the cantaloupe-like hematite continuously ripened and finally formed peanut-like hematite (step 5, Fig. 5f, 6 h). From the appearance of hematite reflection peaks at 2 h, peak intensity gradually increased, indicating the higher crystallinity of peanut-like hematite compared with other hematite types. Apart from the factors mentioned above, the attraction of crystallographic plane, hydrophobic interaction, and van der Waals forces can also affect the morphology . Thus, additional studies are still needed to reveal the process and evolutionary mechanism of peanut-like hematite.
In summary, a new one-pot hydrothermal approach to obtain peanut-like hematite with a large BET surface area was developed in this study. The results showed that peanut-like hematite was formed by a five-step route consisting of nucleation, aggregation, phase transition, anisotropic growth, and ripening. Hematite with different morphologies can be obtained by properly controlling the hydrothermal conditions. The peanut-like hematite showed good preference and performance for As(V) removal, and the process was in accordance with the pseudo-second-order kinetic model. This study does not only serve as a reference for the synthesis of other hierarchical metal oxides or hydroxides, but also provide an opportunity to study the catalytic and electromagnetic performances of materials with different morphologies.
This study was supported by the National Natural Science Foundation of China (No. 41373114); and the Program of Introducing Talents of Discipline to Universities (No. B06006).
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